Syncthing slow when installed via Docker/Portainer in Open Media Vault; ports are forwarded and Syncthing is connecting via TCP and not by relay, but sync speeds haven't improved. What should I be looking at to fix this?

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Then its most likely not the network thats the bottle-neck.

Suggest you check cpu usage/io load on storage, and verify syncthings database is on fast storage

Also, speeds might be slow if you have many small files, as there is non-trivial overhead per file in terms of book keeping in the database.